Generally, an optically pumped laser comprises an optical system which directs an exciting optical radiation ("pump radiation") into an active laser medium to produce the amount of population inversion necessary for laser action.
A known optical laser pumping system comprises a cylindrical reflector having an elliptical cross section. A flash lamp is positioned along a first focus line of the reflector to focus the radiation emitted by the flash lamp into an elongated laser medium positioned in the other focus line of the reflector.
A known optical dye laser pumping system comprises an excimer laser as pump radiation source and a cylindrical lens for focussing the pump radiation into a cuvette comprising a dye solution as active laser medium. Such a system is simple, however, it has the draw-back that the distribution of the excited (pumped) dye molecules is very uneven because the intensity of the pump radiation decreases, due to absorption, exponentially in the dye solution from a stripe-shaped irradiated surface region of the laser medium toward its opposite side. Thus, the amplification also tends to become uneven which causes a heavy distortion of the intensity distribution in the amplified laser beam. Further, the nonuniform excitation of the dye solution produces a gradient of the index-of-refraction of the solution, because the index-of-refraction of the dye solution decreases essentially proportional to the density of the excited dye molecules. This gradient tends to cause an undesired deflection of the laser beam and a distortion of its wave front. Increasing the depth of penetration of the pump radiation into the dye solution by decreasing the dye concentration of the solution is no acceptable remedy of the above problem, since it would cause a corresponding decrease of the efficiency of the pump radiation.
A known method which secures a uniform distribution of the pump radiation over the cross-section of the laser material is the so-called longitudinal pumping. Longitudinal pumping means that the pump radiation beam is propagated coaxially along the longitudinal axis of the laser medium which is generally of cylindrical shape. A laser beam to be amplified may be introduced into the laser medium along its axis by means of a dichroitic mirror which transmits the pump radiation essentially without damping and reflects the radiation to be amplified as completely as possible. It is known that an excellent beam quality can be obtained by longitudinal pumping, however, it is also known that the amplification obtainable is generally quite low.
An essential step forward was the system developed by D. S. Bethune (Appl. Opt. 20, 1897 (1981)). The Bethune system comprises a prism having a triangular cross-section with a rectangular edge and a longitudinal hole. The pump radiation beam enters through the hypotenuse surface and is reflected at the cathete surfaces. The laser material is positioned in the longitudinal hole and, thus, irradiated from four sides with the pump radiation so that a more uniform distribution of the excited dye moleculs resides. However, the inversion is still less near the axis of the active material than at the circumference thereof, if the laser medium is sufficiently concentrated to guaranty an effective absorption of the pump radiation. Thus, the Betune system suffers from an insufficient pump radiation efficiency and a strong decrease of the inversion from the circumference to the axis of the active laser medium as well as from an uneven angular distribution of the intensity on the circumference of the laser medium.
German patent application disclosure document No. 12,93,932 describes an optical laser pumping system which comprises a reflector in form of an elongated ellipsoid of revolution having a longitudinal axis and first and second focus points. A laser rod extends from the first focus point axially to the adjacent surface of the reflector and a gas discharge lamp is positioned between the other focus point and the adjacent surface of the reflector. The large size and manufacturing costs have prevented any significant practical use of this device.
Swiss patent No. 419,371, French patent No. 1,471,267, and U.S. Pat. No. 3,675,156 describe optically pumped laser systems employing an optical pump cavity including two facing, internally reflective frustro-conical elements in which an elongated laser medium and an elongated pump lamp are respectively axially oriented. The two conical elements may be connected by an internally-reflective cylindrical element. While these optical systems transfer most of the pump radiation emitted by the pump lamp on the elongated laser element, the distribution of the pump radiation on the circumferencial surface of the laser element is not very well defined, mainly because of the extended longitudinal and radial dimensions of the pump radiation source.